Bearing performance is measured by bearing load capacity, stiffness and damping. Load capacity denotes rotor support limit during operation. Stiffness denotes the restoring force imparted to a shaft by the bearing when the shaft is deflected from its geometric axis. High bearing stiffness is desirable in order to maintain accurate shaft positioning as loads are applied to the shaft. Damping refers to the rate at which vibrational energy is removed from the rotating bearing-shaft system. Damping is necessary to attenuate vibration of the shaft at speeds near or at the resonant frequency of the bearing-shaft system.
Magnetic bearings can provide superior performance over mechanical bearings such as ball bearings. Magnetic bearings have low drag losses, high stiffness and damping and moderate load capacity. Unlike mechanical bearings, magnetic bearings do not have to be lubricated.
There are two types of magnetic bearings; active and passive. Active magnetic bearings are characterized by an iron rotor that is surrounded by electromagnetic coils (i.e., a stator) and positioning/sensing electronics. When the coils are energized, attractive forces between the iron rotor and coil cause the rotor to be suspended. Stiffness and damping are controlled by the electronics.
Passive magnetic bearings do not utilize electronic controls. Instead, conventional passive magnetic bearings are characterized by two sets of permanent magnets. One set of magnets is employed in the rotor, and the other set of magnets is employed in the stator. A conventional passive magnetic bearing is disclosed in U.S. Pat. No. 3,614,181 issued to Meeks on Oct. 19, 1972. Secured to a shaft are a plurality of radially-polarized magnets, which are arranged in alternating polarity. The shaft is surrounded by radially polarized ring magnets also arranged in alternating polarity. Resulting are uniform radial repulsive forces that cause the shaft to be suspended. The shaft can be rotated by a minimal amount of force. However, according to Earnshaws Theorem, total permanent magnet levitation is inherently unstable and, hence, not practical for use in bearing systems. Passive magnetic bearings have low losses and a simple design.
Passive magnetic bearings can also be made of superconducting materials. Superconductors are classified as being either Type I or Type II. When cooled below a critical temperature T.sub.c, Type I and Type II superconductors have the ability to screen out all or some of the magnetic flux applied by an external source. Type I superconductors exhibit total flux expulsion for applied magnetic fields less than some critical field H.sub.c and critical temperature T.sub.c. This is believed to be caused by persistent currents that flow at the surface of the Type I superconductor. The expulsion of flux from a Type I superconductor is known as the "Meissner Effect." When expelled, the flux flows around the superconductor, providing a lifting force. This lifting force causes a magnet to be levitated above a Type I superconductor that is held stationary. For applied magnetic fields above the critical field H.sub.c, the superconductive properties are lost.
Type I superconducting materials are used for thrust bearings. For examples of superconducting bearings made of Type I superconductors, see U.S. Pat. No. 3,493,274 issued to Emaile et al. and U.S. Pat. No. 3,026,151 issued to Buchhold. However, magnetic bearings made of Type I superconducting materials are thought to experience rotor stability problems. In order to stabilize the rotors of these systems, the bearings generally employ either a mechanical rotor support (e.g., Buchhold) or dished structures that provide a gravitational minimum (e.g., Emaile et al.).
Type II superconductors also exhibit total flux expulsion for applied magnetic fields less than a first critical field H.sub.C1. For applied magnetic fields in excess of a second critical field H.sub.C2, the superconductivity is lost. In between critical fields H.sub.C1 and H.sub.C2, however, Type II superconductors exhibit partial flux exclusion. Partial flux exclusion is believed to be caused by inhomogeneities (e.g., pores, inclusions, grain boundaries) inside the Type II superconductor. When the magnetic field is being induced into the superconductor, the superconductor offers resistance to change or displacement of this induced magnetic field. Some of the magnetic flux lines become "pinned" within the superconducting material. This phenomenon is known as "flux-pinning." The remaining flux lines are repelled by the flux lines pinned in the superconductor. This repulsion causes levitation. Thus, levitation does not arise from the Meissner effect. Instead, levitation occurs because the superconductor behaves more like a perfect conductor than a Meissner conductor.
Type II superconducting materials are more commonly used for rotating bearings. Thrust bearings are created by levitating a magnet above a disk made of a Type II superconductor. Furthermore, journal bearings can be made by levitating a magnet inside a ring of Type II superconducting material. Due to flux pinning, a bearing made from Type II superconducting material, such as YBa.sub.2 Cu.sub.3 O.sub.X, displays far greater stability than a bearing made of a Type I superconducting material. For an example of a Type II superconducting bearing, see Iannello et al., "Superconducting Meissner Effect Bearings for Cryogenic Turbomachines", Defense Technical Information Center, Alexandria, Va., no. AD-A209-875 (May 18, 1989). A cylindrical magnet is placed within a hollow shaft, which is made of a Type II superconducting material. In one embodiment, a neodymium rare-earth magnet is placed within a shaft composed of YBa.sub.2 Cu.sub.3 O.sub.X superconducting material.
The load capacity and stiffness of the superconducting bearing are dependant on the flux density and flux density gradient at the superconducting surface. Flux density is defined as the amount of magnetic flux per unit area. Flux density gradient is defined as the change of flux density over distance normal to the magnetic surface. For the prior art superconducting bearing shown in FIG. 1, flux lines 2 flow between poles of a permanent magnet 4. The magnet 4 is a distance x from a superconducting member 6, which is made of a Type II superconductor. Some of the flux lines 2 are pinned in the superconducting member 6. Flux pinning gives flux repulsion, which causes the magnet 4 to be levitated above the member 6. When an external force F is applied to the magnet 4, the magnet 4 is forced towards the member 6. As the magnet 4 is forced closer to the surface of the member 6, the resulting repulsive force (i.,e., restoring force) is increased. This increase is due to the flux density gradient. Bearing stiffness K is defined as K=dF/dx. Thus, as the flux density gradient is increased, the bearing becomes stiffer. Further, axial and radial load capacity is increased as the flux density between the magnets and the superconductor is increased.
Therefore, it is an object of the present invention to increase load capacity and stiffness of a superconducting bearing.